The invention pertains to an optical coupler including at least one input waveguide, a coupling region, and a plurality of output waveguides.
U.S. Pat. No. 5,136,671 is directed to an Nxc3x97N integrated optical interconnection apparatus having two such couplers. In this case, the apparatus is so called a star coupler. In the description of U.S. Pat. No. 5,136,671, it is explained that optical switching, multiplexing, and de-multiplexing have been-accomplished in the past by using an interconnection apparatus having a plurality of closely spaced input waveguides communicating with the input of a star coupler. The output of the star coupler communicates with an optical grating having a series of optical waveguides, each of the waveguides differing in length with respect to its nearest neighbour by a predetermined fixed amount. The grating is connected to the input of a second star coupler which form the outputs of the switching, multiplexing, and de-multiplexing apparatus.
It is recognised in U.S. Pat. No. 5,136,671 that, in order to achieve high efficiency power transfer between a relatively large number of input ports and a relatively large number of output ports, the input and output waveguides connected to the star couplers must be closely spaced in the vicinity of the star couplers. This causes a significant degree of mutual coupling between those adjacent input and output waveguides, producing increased undesirable cross-talk between the channels of the device and decreased efficiency in transferring optical power from the selected input ports to selected output ports of the apparatus.
The star couplers described in U.S. Pat. No. 5,136,671 have a dielectric slab which forms a free space region having two curved, preferably circular, boundaries. The input waveguides and the waveguides in the grating are connected to the free space region in a substantially uniform fashion along the boundaries.
U.S. Pat. No. 4,786,131 is directed to an Mxc3x97N (star) coupler having a planar waveguide having a pair of opposed edges that serves as entrance and exit facets xe2x80x9cfor introducing and extracting electromagnetic radiation from said waveguide.xe2x80x9dThe planar waveguide is structured to confine the radiation propagating therein in a single mode in its depths without confining it in its width so that, in its width, radiation propagates as an expanding wavefront.
For the sake of completeness, it is noted that European patent application 0 717 295 discloses an Mxc3x97O multiplex-/de-multiplex device including two evanescent wave couplers each having an array of fused optical fibres. The phase distribution at the output of such couplers does not describe a circular or elliptic phase front, which are therefore less suitable for use in AWGs. This is especially true for wavelength de-multiplexers designed to operate at a broad wavelength range.
According to the above prior art documents, although the phase distribution at the output plane of the star couplers can be adequately matched to the output waveguides, the amplitude distribution cannot. As a result of such amplitude mismatch, a considerable amount of the electromagnetic radiation will be coupled in the areas between the output waveguides, in turn resulting in insertion losses and additional cross-talk, especially when applied in arrayed waveguide gratings.
It is one of the objects of the present invention to provide an improved optical coupler, wherein the amplitude distribution substantially matches the output waveguides.
To accomplish the above and other objects, the coupler according to the first aspect is characterised in that the coupling region comprises a plurality of coupled waveguides which, over at least part of their lengths, diverge with respect to each other in the propagation direction of electromagnetic radiation launched in the input waveguide.
It is preferred that the width of at least some of the waveguides in the coupling region also increases, preferably gradually, and/or that the width of the gaps between the waveguides in the coupling region is at least substantially constant.
As will be explained below, coupling between the waveguides is enhanced considerably if at least some of the waveguides in the coupling region includes a section having a width that is less than the critical width of the waveguide at the wavelength(s) at which the coupler is designed to operate.
In a further preferred embodiment, the centre lines of at least some of the gaps between the waveguides in the coupling region follow the lines of a Gaussian diffraction pattern in accordance with the following set of equations (E1) or a linearised version thereof:                                           w            ⁡                          (              z              )                                =                                    w              k                        ⁢                                          1                +                                                      (                                          α                      ⁢                                              xe2x80x83                                            ⁢                      z                                        )                                    2                                                                    ;                  α          =                                    (                              λ                /                                  n                  eff                                            )                                      π              ⁢                              xe2x80x83                            ⁢                              w                o                2                                                    ;                  R          =                      z            ⁡                          (                              1                +                                                      (                                          1                                              α                        ⁢                                                  xe2x80x83                                                ⁢                        z                                                              )                                    2                                            )                                                  E1      
where z is the longitudinal propagation position; w(z) is the z-dependent lateral position of the central line of the kth gap; wk is the position of the centre of the kth gap at z=0; w0 is the beam waist at z=0; xcex is the wavelength in vacuum, Neff is the effective index and R is the radius of curvature of the phase front.
In a coupler designed using the above equations E1, the sum of the widths of the waveguides and gaps gradually, increases in the propagation direction, and insertion loss and cross-talk are further reduced.
The amplitude distribution and, hence the distribution of power, over the output waveguides is further modified by adjusting the positions in the propagation direction, where centre lines of the gaps between the waveguides in the coupling region start to follow the lines of a diffraction pattern. For example, the power distribution is equalised so as to obtain a very effective power coupler. The above modification is preferably done by means of the following set of equations (E2) or a linearised version thereof:                               w          ⁡                      (            z            )                          =                  {                                                                                                                w                      k                                        ,                                                                                                            ⁢                                                                  for                        ⁢                                                  xe2x80x83                                                ⁢                        z                                             less than                                               z                        k                                                                                                                                                                                                            w                        k                                            ⁢                                                                        1                          +                                                                                    [                                                              α                                ⁡                                                                  (                                                                      z                                    -                                                                          z                                      k                                                                                                        )                                                                                            ]                                                        2                                                                                                                ,                                                                                                            ⁢                                                                  for                        ⁢                                                  xe2x80x83                                                ⁢                        z                                             greater than =                                              z                        k                                                                                                                  ;                          α              =                                                (                                      λ                    /                                          n                      eff                                                        )                                                  π                  ⁢                                      xe2x80x83                                    ⁢                                      w                    o                    2                                                                                                  E2      
Since the gaps are parallel to the flow direction of the light, there is no reflection of the light to the side wall (read gaps here) of the waveguide. Since no reflection occurs to the side walls, there will be no interference due to the presence of multiple light paths, thereby maintaining the phase distribution of the light similar to a free diffraction region. The presence of the gaps, however, ensures that the amplitude distribution is affected though.
In a preferred embodiment, when electromagnetic radiation of a wavelength at which the coupler is designed to operate is launched in one of the inputs, the coupler generates an amplitude distribution, which exhibits, in a lateral direction, a plurality of peeks and wherein the output waveguides are positioned at the lateral positions of these peaks. Thus, the amount of radiation coupling into the gaps between the waveguides as well as cross-talk are further reduced.
In general, it is preferred that all the above-mentioned waveguides in the coupler according to the present invention are planar waveguides.
The invention also pertains to an Arrayed Waveguide Grating (AWG), also known as inter alia Phasar, Phaseur, and Waveguide Grating Router, having the present coupler. The advantages of the coupler are especially noticeable in AWGs, since even small deviations from the amplitude and phase distributions for which such a device was designed result in substantial losses or render it inoperative all together.